Petcock Refurb

[Guest]

I had some time yesterday so I decided to try my hand at refurbishing a petcock.
I understand that the most common cause for failure is diaphragm fatigue. The 30 year old diaphragm develops holes from the stress of on/off vacuum. So the trick to fixing it is to separate the diaphragm from its attached o-ring in order to replace the diaphragm with a suitable substitute.

Here is the unmolested diaphragm.


I drilled out the "rivet" holding the pieces together and separated the components.


I then counter sunk the top piece and threaded the bottom piece so that I could reassemble.


Now the only question is what material to replace the diaphragm with?

Cloth-inserted nitrile is likely the best thing to use but I can't seem to find that without buying too much of it.

The simplest solution is to use a section of nitrile glove but who knows how long that will last.

[Adler]

No interest in restoring a petcock? I figured with the cost of replacement and the frequency of failure this would be a hot topic.

[tkent02]

There is interest, did you find a suitable material for the diaphragm?

[ChicagoBob]

You'd need something thin, but durable. A piece cut from a Soviet-era Bulgarian condom?

Serously, you can get replacement diaphrams from several sources, and they work OK. You can also buy old GS petcocks on eBay pretty cheap. I got some for less than $5, and one had a good diaphram that I'm still using a year later.

[Souske]

How much material is too much? Are they making you buy a full yard?

[Adler]

Good morning Adler.

I am interested in any repair you do. (Still want to know how those bikes are coming along).

I have not ever looked at a diaphram so....

As a kid I used to cut inner tubes to make rubber parts to specs. Perhaps an old tube from a motorcycle or car would be too thick?

One from a bicycle might have enough circumference and be of a suitable thickness.

Resistance to gasoline and ethanol is another ball game altogether.

DP

I am taking my sweet time on the bikes.

A bicycle tube might be thin enough but there is no way it would have the fuel resistance.

I found this place. http://www.rubbersheetroll.com/nitri...h-inserted.htm
They sell sheets of .032" X 36.00" X 36.00" cloth inserted nitrile. I wish they would list prices online but I guess I will have to call or email. We will see if they ship to Canada.
Do you think 0.032" will be thin enough?

[Adler]

I've done a little more experimenting on this topic.

First I tried a sheet of nitrile glove material backed with the original diaphragm. It leaked around the edges.

Then I tried using just nitrile glove material. Again, it leaked around the edges.

Finally, I tried using just nitrile glove material and I cut a gasket for the edge. That held vacuum but the glove is very thin and I suspect it would fail before too long. even if it does fail after a year or so, its easily replaceable so that is already a decent alternative.

The next thing to try is all three. Nitrile, old diaphragm for support and gasket to seal the edge.

Ill try to get pictures to help you visualize what I have done.

[Adler]

Here are some pictures from after I rigged it up to the bike to test it (it worked).









You can see some deformation of the nitrile so I am not too sure that this would last for very long.

It will be quite some time before I am able to test this in practice because my bike is still not road ready.

[snicrep5]

due to all the reported rebuild failures, i just bought one off ebay in the 30-40 dollar range. im a little impatient and just want to getr it going.